Improve StringIndexOutOfBoundsException throwing.

Better, more-consistent detail messages, more regular naming, plus use of
the one true bounds-checking idiom (including reversal of sense to reduce
unnecessary indentation).

(The lack of an inlining JIT means that I couldn't factor the bounds checking
out, but at least we're working towards full duplicates rather than
near-duplicates!)

Change-Id: I59ded68169067305e747d30801c79a4c13f521b2